Preparation of photographic emulsions with polyvinyl acetate having a high acetyl content



Patented Jan. 14, 1 947 PREPARATION OF PHOTOGRAPHIC EMUL- SIONS WITH POLYVINYL ACETATE HAV- IN G A HIGH ACETYL CONTENT Wesley G. Lowe, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a

corporation of New Jersey No Drawing. Application January 22, 1944,

Serial No. 519,338

7 Claims.

This invention relates to lacquer-type photosensitive emulsions in which a polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent is employed as the protective colloid for the light-sensitive element therein.

In recent years the application of light-sensitive material to metal or other surfaces, especially where such surfaces are large, irregular, or curved, in order to print a design thereon has become of interest. This expedient is, for instance, useful in marking metal sheets in the production of airplane wings and bodies. The most convenient method of applying these lightsen-sitive materials is by spraying or brushing, but the usual types of photo-sensitive emulsions composed of gelatin and silver halide are not particularly satisfactory for this purpose due to the difficulties involved in coating them upon a surface satisfactorily. For instance, gelatin emulsions on spraying show a tendency to foam and temperatures above normal may need to be employed which is often inconvenient; furthermore, gelatin emulsions take a considerable time to dry. Speculative statements have been made in the prior art concerning the use of polyvinyl acetates having a high vinyl acetate content for use as the carrier for silver halides in photographic emulsions. There has been no attempt, however, to teach the proper conditions of operation or the disadvantages of some types of these materials and the advantages of other types. Also, there has been no teaching of the value of these materials in forming emulsions useful in lacquering or coating operations. such as applied to surfaces by brushing or spraying.

One object of my invention is to provide a method of preparing lacquer-type emulsions which are particularly suitable for applying to irregular surfaces, such as of metal or other substances so that designs may be printed thereon. Another object of my invention'is to provide an emulsion which upon coating and drying provides a layer which is permeable to processing solutions but which swells little and ha good wet adhesion. Other objects of my invention will appear herein. I have found that the polyvinyl acetates having a vinyl acetate content of 80 to 90 per centhave the property when used as the protective colloid in photographic silver halide emulsions of forming emulsions which have a lacquer-like character and which are especially suitable for coating by spraying or brushing. I have found that by using such protective colloids, rapidly drying emulsions can be formed which are eminently suitable for coating large surfaces,

which because of their size can only be readily dried by leaving the surface exposed to the atmosphere at room temperature. I have found that by using polyvinyl acetates having a vinyl acetate content of 80 to 90 per cent as the carrier for silver halide in a lacquer-type emulsion containing a volatile solvent, water, and preferably a high boiling solvent, that the resulting composition may be readily applied to a surface either by brushing er spraying, and that the coating formed has good abrasion resistance while wet.

In its broadest aspects, the composition of my invention is prepared by forming the silver halide in a solution of the polyvinyl acetate and then adjusting the proportion of solvent so as to assure a readily flowable emulsion which may then be applied to the surface upon which a lightsensitive coating is desired.

Polyvinyl acetate of 80 to 90 per cent vinyl acetate content can be obtained by hydrolyzing to the required degree a polyvinyl acetate of higher vinyl acetate content. The molecular weight of the initial polyvinyl acetate may be up to 60.000 or even more and examples of suitable polyvinyl acetates are found both in the low viscosity, low molecular weight types, and in the high viscosity, high molecular weight types. For instance a polyvinyl acetate having a viscosity of 7 seconds may be hydrolyzed and employed; on the other hand, polyvinyl acetate having a viscosity of seconds may be used after suitable hydrolysis. The lower viscosity materials form emulsions having lower wet adhesion and lower abrasion resistance than the higher viscosity material, but, on the other hand, the low viscosity materials give emulsions in which the grains of silver halide disperse more readily.

The initial polyvinyl acetate can be hydrolyzed to the to per cent vinyl acetate stage by dissolving in ethyl a cohol and adding the necessary quantity of dilute aqueous causticsoda at room temperature with stirring. The caustic soda is neutralized and the hydrolysis is stopped at a stage depending upon the quantity of caustic soda used, the reaction being usually run for about 30 minutes. Thus in the case of 7-second polyvinyl acetate, atisfactory hydrolysis can be carried out by adding 200 cc. of ethyl alcohol containing 4.65 grams of caustic soda to a solution of grams of the polyvinyl acetate in 800 cc. of ethyl alcohol slowly and with stirring. The reaction is complete in about 30 minutes at room temperature. The solution of hydrolyzed acetate so obtained, the caustic soda having been neutralized, can be used as it stands, forming the photographic 3 emulsion, and there i no need to isolate the product. An acid method of hydrolysis could be used if desired, but it is considerably slower and is not so easily stopped at the required degree of hydrolysis.

The lacquer-typ'e emulsion can be formed from the polyvinyl acetate in various ways which will be apparent to those skilled in the art. The preferable method is to dissolve silver nitrate and soluble halide separately in aqueous ethyl alcohol and to add the solution simultaneously and with stirring to an aqueous ethyl alcohol solution of the polyvinyl acetate, although it is possible to first dissolve the soluble halide in the polyvinyl acetate solution and add the silver nitrate solu tion thereto. The polyvinyl acetate solution should always contain sufilcien't alcohol to keep it dissolved. (A 1:1 mixture of water and alcohol is usually suitable.) It is essential that thorough mixing should take place when combining the silver nitrate and soluble halide in order to avoid aggregation of the silver halide grains. Ammonia may be added if desired providing the quantity is insuflicient to cause fog, the amount depending upon the temperature, the length of the ripening of the emulsion, and the nature and excess of the soluble halide used.

The emulsion so obtained is then ripened by allowing it to stand at roomv temperature, after which more of the polyvinyl acetate is usually added. The emulsion may then be washed by pouring into water and precipitating, and after settling, the precipitated emulsion is separated and. dissolved in a suitable solvent such as ethyl alcohol, preferably with the addition of a quantity of a compatiblehigh boiling solvent, for example butyl alcohol or a Cellosolve, to impart more desirable coating properties, The high boiler may constitute -50% of the liquid materials present in. the emulsion. It is ordinarily preferable, however, to employ an amount of high boiler within the range of -25% of the liquid materials present. The sensitivity of the emulsion can be improved at this stage by digesting it at 40 to 60 C. or above and a sulphur sensitizer suchas allyl isothiocyanate, isopropylthiocarbonic ester, a thiourea or a thiourethane can he added. The sulphursensitizer can be added dissolved in a suitable solvent, and the optimum quantity of solvent should be determined by trial.

Although ethyl. alcohol is given as bein employed in preparing the emulsion containing low boiling solvents, other water-soluble solvents can be employed in their stead. For instance, acetone is admirably suited for this purpose. ter of fact, any water-soluble low boiling solvent which does not have a detrimental effect upon the silver halide and which is a solvent for the polyvinyl acetate may be employed.

Emulsions obtained as described can be applied directly to surfaces of cellulose esters, wood or anodized aluminum. In. some cases, it may be desirable to first apply an undercoating of a cellulose nitrate lacquer, particularly on steel, zinc, or metal surfaces. Good adhesion is obtainable and the emulsion layers dry rapidly such as in minutes at room temperature. The coatings may be readily developed by the following developin and iodizing solutions:

Developing solution Amidol -grams 8 Sodium sulphite (crystalline) do 80 Potassium bromide do 4.16

Water to c, c 1,000

As a mat- 4 The bromide content should be adjusted to give a convenient time of development.

Iod'izing solution Potassium iodide grarns 30 Sodium metabisulphite' do 50 Water to c. 0.. 1,000

By subjecting to treatment with the solutions as known for the developing of emulsions in the photographic art, after exposure, an image is formed upon the surface to which the emulsion is applied. The processing of the emulsion is a matter of choice with the individual operator, the solutions given being merely illustrative, these solutions having been used and found to Work satisfactorily.

The preparations of emulsions in accordance with my invention are illustrated by the following examples:

EXAMPLE 1 6 grams of partially-hydrolyzed polyvinyl acetate prepared from a polyvinyl acetate having a viscosity of 60 seconds, and which have been hydrolyzed down to 81.3 per cent vinyl acetate content was dissolved in a mixture of c. c. of acetone and 45 c. c. of water. This solution was warmed to a temperature of 85 F., .and the following solutions were allowed to flow into it simultaneously with good stirring over a period of 5 minutes:

Solution A 5 grams of silver nitrate made up to 10 c. c. with water.

Solution B 4.8 grams of potassium bromide, 0.35 grams of potassium iodide made up to 13.5 c. c. with water.

Both of these solutions were held at F. At the endof the precipitation, there was added 2.4 grams of the partially-hydrolyzed polyvinyl acetate dissolved in 10 c. c. of water and 10 c. c. of acetone, and the emulsion was ripened for 20 minutes at 104 F. Theemulsion was washed by pouring into water with stirring; The precipitate was pressed to a wet weight of 42 grams and dissolved by adding 15 c. c. of Water, 60 c. c. of acetone and 10 c. c. of ethyl Cellosolve with stirring. The emulsion was optically sensitized by thoroughly incorporating therein 2 mg. of 2,3-

diethyl 4f methyloxathiazolo carbocyanine iodide, and the emulsion was coated and dried onto a surface. The emulsion coating was exposed and was then developed in the following developing solution:

Development was found. to be slow but clean. The emulsion fixed slowly in an ordinary fixin bath but was fixed rapidly in a solution of, sodium thiosulphate to which alcohol had been added, or in an aqueous solution of potassium cyanide. It was also found that ammonium thiosulphate solution. fixed the emulsion at a satisfactory rate.

EXAMPLE 2 The following solutions were prepared:

Solution A c. c. per cent solution in ethyl alcohol of polyvinyl acetate of 80 to 90 per cent vinyl acetate content 800 Ethyl alcohol 800 Water 800 Solution B Silver nitrate rams 200 Ammonium hydroxide sufiicient to dissolve precipitate Water to make up to 600 c. 0.

600 c. c. of ethyl alcohol were added to solution B just before using, and solutions B and 0 each at C. were run separately into solution A, also at 20 C., so that each addition takes 10 minutes. The addition of solution C is started /2 minute before the addition of solution B, and the latter is thus added for minute after the addition of solution C has been completed. Solution A is vigorously stirred during the mixing, and the jets delivering solutions B and C are preferably below the surface of solution A. 800 c. c. more of a 10 per cent solution of polyvinyl acetate is then added, and the emulsion is stirred for one minute and then precipitated by pouring it into 56 litres .of Water. The emulsion was allowed to settle for By volume Emulsion 50 n-Butyl alc Ethyl acetate 22.5 Ethyl lac 2.5

Upon coating out upon a surface a light-sensitive coating is obtained of good speed.

I claim:

,1. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of to per cent and a silver halide dispersed therein in a liquid consisting of water, a low boiling water-soluble solvent for the polyvinyl acetate and 0 to 50 per cent of a high boiling compatible liquid.

2. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent and a silver halide dispersed therein, in a liquid consisting of Water, a low boiling watersoluble solvent for the polyvinyl acetate and 10 to 25 per cent of a high boiling compatible liquid.

3. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent and a silver halide dispersed therein, in a liquid consisting of water, ethyl alcohol and 0 to 50 per cent of a high boiling compatible liquid.

4. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent and a silver halide dispersed therein, in a liquid consisting of water, acetone and 0 to 50 per cent of a high boiling compatible liquid.

5. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent and a silver halide dispersed therein in a liquid consisting of water, a low boiling water-soluble solvent for the polyvinyl acetate and 10 to 25 per cent of ethyl Cellosolve.

6. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent and a silver halide dispersed therein in a liquid consisting of water, acetone, and 10 to 25 per cent of ethyl Cellosolve.

7. A lacquer-type light-sensitive silver halide emulsion adapted to spraying or brushing essentially consisting of a solution of polyvinyl acetate having a vinyl acetate content of 80 to 90 per cent and a silver halide dispersed therein in a liquid consisting of water, ethyl alcohol, and 10 to 25 per cent of ethyl Cellosolve.

WESLEY G. LOWE. 

